1. Field of the Invention
The present invention relates to a technique of supplying molten glass. The invention more specifically relates to an improvement in a molten glass supply device that supplies molten glass exhibiting high viscosity such as sheet glass for a liquid crystal display from a melting furnace to a forming device and an improvement in a technique of producing a glass product such as sheet glass for a liquid crystal display by supplying the molten glass from the melting furnace.
2. Description of the Related Art
In recent years, there has been a rapidly increasing demand for a glass substrate for a flat panel display such as a liquid crystal display (LCD), and an electroluminescent display (ELD), cover glass for various image sensors such as a charge coupled device (CCD), a life-size magnification, solid-state contact image sensor (CIS), and a CMOS image sensor, and a glass substrate for a hard disk and a filter.
Glass for the items described above and equivalent items is high viscosity glass, while glass for items such as a glass panel or a glass funnel for a cathode ray tube (CRT), window sheet glass, a vase, and tableware and equivalent items is low viscosity glass. These kinds of glass have considerably different characteristics.
Now, let us consider high viscosity glass, non-alkali glass for a liquid crystal display and typical low viscosity glass, soda-lime glass for a container as examples. As shown in FIG. 5, as can be seen from the characteristic curve A of the glass for the liquid crystal display, the viscosity is not suitably lowered until the temperature reaches an extremely high temperature region of about 1400° C. or higher, and the smooth flow of molten glass in the molten glass supply device that will be described cannot be maintained. Meanwhile, the characteristic curve B of the soda-lime glass shows that the viscosity is suitably lowered at a temperature of about 1200° C. or lower. More specifically, in the glass for the liquid crystal display (characteristic curve A), the viscosity is 1000 poise or less at a temperature of about 1460° C. or higher. Meanwhile, in the soda-lime glass (characteristic curve B), the viscosity is 1000 poise or less at a temperature of about 1180° C. or higher.
In general, when the high viscosity glass has a viscosity of 1000 poise, the corresponding temperature is at least 1350° C. The temperature is 1420° C. or higher for particularly high viscosity glass. When the low viscosity glass exhibits a viscosity of 1000 poise, the corresponding temperature is 1250° C. or lower. The temperature is 1200° C. or lower for particularly low viscosity glass. Therefore, the high viscosity glass and the low viscosity glass can be distinguished based on the relation between the temperature and the viscosity.
Meanwhile, in producing the above-described items of high viscosity glass, high viscosity glass is supplied in the form of molten glass to a forming device and formed into a substrate of sheet glass in the device. Therefore, when these items are produced, a melting furnace serving as a supply source of molten glass and a molten glass supply device including a supply path for supplying molten glass let out from the furnace to the forming device are employed.
In the molten glass supply device, the viscosity of the molten glass must be lowered in order to smoothly supply the molten glass from the melting furnace to the forming device through the supply path. In this case, as can be clearly understood from the foregoing comparison between the characteristic curves A and B shown in FIG. 5, the temperature of high viscosity glass must be much higher than that of low viscosity glass so that these kinds of glass have the same low viscosity.
Consequently, it is more difficult for a molten glass supply device for high viscosity glass to smoothly flow molten glass than a molten glass supply device for low viscosity glass, and therefore the former device should be adapted to less impede the fluidity of molten glass. Therefore, as disclosed by Japanese Patent Laid-Open Publication No. 2000-185923 (FIG. 2), when high viscosity glass is used, for example the kind of device that supplies molten glass to the forming device from the melting furnace through a single supply path (hereinafter also referred to as “single feeder”) is employed.
Meanwhile, in Japanese Patent Publication No. Sho 48-17845 and Japanese Patent Laid-Open Publication Nos. Sho 62-176927, Hei 6-24752, and 2000-313623, each of disclosed devices supplies molten glass let out from a melting furnace to a plurality of branch paths through a distribution chamber (hereinafter also referred to as “multi-feeder”). The multi-feeder is, however, for the low viscosity glass rather than the high viscosity glass described above. More specifically, Japanese Patent Publication No. Sho 48-17845 discloses “window glass” while Japanese Patent Laid-Open Publication No. Sho 62-176927 discloses “glass gob” and “glass for container.” Japanese Patent Laid-Open Publication No. Hei 6-24752 includes a description of “a glass bottle” and a description of a glass composition in Table 1 that clearly suggests the low viscosity glass. Japanese Patent Laid-Open Publication No. 2000-313623 has a description of “a glass bottle and glass ware,” and therefore the multi-feeder disclosed in each of the documents is clearly directed to the low viscosity glass.
A molten glass supply device for high viscosity glass must maintain molten glass in its melting furnace at extremely high temperatures (1500° C. or higher for example) using heating means. In the conventional device having a melting furnace for each supply path, however, when molten glass is supplied to a plurality of forming devices through the plurality of supply paths, heat is radiated from the entire periphery of the plurality of melting furnaces, and therefore the amount of radiated heat per unit area is inevitably large. In addition, the total heat radiation area can be large; in other words, the total amount of heat radiation can be large, which increases the cost required for heating to an improperly high level.
Furthermore, a refractory material (such as firebricks) for melting furnaces is eroded as it contacts the molten glass at high temperatures. This is for the following reasons. There are different kinds of such refractory materials available in the low temperature range and a refractory material less susceptible to erosion by contact with molten glass can be selected relatively easily for the low temperature range. In contrast, the refractory material can easily be eroded by contacting high temperature molten glass and the refractory material that can resist high temperatures in the high temperature range is limited to such a high zirconia content material. Consequently, the flexibility in choice is limited or the choice of such a less susceptible to erosion refractory material is impossible.
Therefore, as in the conventional device, if a melting furnace is provided for each supply path in a molten glass supply device for high viscosity molten glass, the entire inner wall surfaces of the plurality of melting furnaces substantially come into contact with molten glass as the glass is supplied through the plurality of supply paths to the plurality of forming devices. Consequently, the amount of erosion foreign material in the molten glass coming into the supply paths or the amount of heterogeneous glass produced due to erosion increases. The erosion foreign material or heterogeneous glass can lower the grade of glass items produced by the forming devices, and the yield can be lowered.
Meanwhile, a molten glass supply device for low viscosity glass needs only to maintain molten glass in its melting furnaces at temperatures much lower than the temperatures for the case of high viscosity glass described above. Therefore, even if the area of heat radiation is large, since the heat radiation amount per unit area is small in this case, the total heat radiation amount is not excessive or the heating cost is not improperly raised. The temperature of the low viscosity glass does not depart from the low temperature range when it is supplied to the forming devices from the melting furnaces. Therefore, the erosion of the melting furnaces can be avoided for the above reasons. Therefore if the contact area between the inner wall surfaces of the melting furnaces and the molten glass is large, the grade of the formed items is not lowered or the yield is not lowered due to erosion foreign materials.
In view of the problems associated with the excessive heat radiation and the erosion foreign materials, the use of the conventional multi-feeder to supply low viscosity glass to forming devices from melting furnaces cannot be advantageous. Meanwhile, the use of the multi-feeder for low viscosity glass whose fluidity is incomparably higher than high viscosity glass is advantageous, for example, in mass production and other purposes. This is why the multi-feeder is used for supplying low viscosity glass today.
More specifically, the disadvantages associated with excessive heat radiation and erosion foreign materials are specific to molten glass supply devices for high viscosity glass. However, in the field of producing glass products made of high viscosity glass, the problems about the heat radiation or erosion foreign materials are not even recognized as problems today. This is because in the field of high viscosity glass, it is generally believed that once the use of the single feeder as the essential configuration is given up, the fluidity of molten glass would be lowered, the forming operation using the forming device could not smoothly be performed and resulting products must have much noticeable defects. Therefore, the possible improvement at best is to modify the conventional single feeder in various manners in order to supply molten glass to the forming devices in the optimum state.
For the foregoing reasons, in the conventional molten glass supply device for high viscosity glass, no countermeasure has been taken in order to solve the problems of the heat radiation or the erosion foreign materials in the melting furnaces.